This invention relates to a band-pass filter for removing noise from PPM (Pulse Position Modulation) binary signal read from a record medium by a reader and a device provided with such a band-pass filter, particularly to a band-pass filter for a PPM signal having suitably set low and high band cutoff frequencies which determine filtration characteristics.
A band-pass filter has been adopted in a measuring equipment and a signal processing device so as to extract, from a signal from a signal source or a signal input device, only components lying within a specified frequency band. Adoption of such a filter enables production of highly reliable equipments and devices by removing unnecessary frequency component noises.
In recent years, there have been provided silver-halide films having magnetic tracks formed by applying magnetic powder in the form of a strip such that they correspond to image recording areas of respective frames. On the other hand, a camera is known which includes a built-in recording head for recording necessary exposure data which were converted into binary data according to a PPM system along the magnetic track. Further, a camera and a film image reproducing apparatus are known which use the exposure data recorded during the photographing operation if necessary. Such exposure data are used to discriminate exposed and unexposed frames against each other when a film in a cartridge which was rewound after being used halfway is wound to a first unexposed frame. In a film image reproducing apparatus or the like for optically reading a film image to reproduce it on a monitor, the exposure data may be read while the film image is read from the exposed film set in a film feeder and used to determine a suitable reproduction method of the read film image (e.g. vertical/horizontal conversion, zooming, panning, etc.).
In the existing equipments and devices, the cutoff frequencies of the filter are generally set only considering the frequency components of the input signal which are to be extracted.
Since the exposure data are written or read while the film is being fed in the existing camera and film image reproducing apparatus, the cycle and phase of the binary signal may not always be stable as in the existing equipments and devices. Accordingly, there is a possibility that the filtration characteristics set only considering the frequency components of the input signal to be extracted deter a preferable signal extraction during the data presence discrimination or the reproduction of the data content. Particularly, the influence of such filtration characteristics is eminent in the case that the exposure data influence their frequency components depending upon their writing and reading speed.